Editing IPCC:AR6/SYR/Longer-Report (section)

==== 3.1.3 The Likelihood and Risks of Abrupt and Irreversible Change ====

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'''The likelihood of abrupt and irreversible changes and their impacts increase with higher global warming levels (''' '''''high confidence).''''' As warming levels increase, so do the risks of species extinction or irreversible loss of biodiversity in ecosystems such as forests ( ''medium confidence'' ), coral reefs ( ''very high confidence'' ) and in Arctic regions ( ''high confidence'' ). Risks associated with large-scale singular events or tipping points, such as ice sheet instability or ecosystem loss from tropical forests, transition to high risk between 1.5°C to 2.5°C ( ''medium confidence'' ) and to very high risk between 2.5°C to 4°C ( ''low confidence'' ). The response of biogeochemical cycles to anthropogenic perturbations can be abrupt at regional scales and irreversible on decadal to century time scales ( ''high confidence'' ). The probability of crossing uncertain regional thresholds increases with further warming ( ''high confidence'' ). { ''WGI SPMC.3.2, WGI Box TS.9, WGI TS.2.6; WGII Figure SPM.3, WGII SPM B.3.1, WGII SPM B.4.1, WGII SPM B.5.2, WGII Table TS.1, WGII TS.C.1, WGII TS.C.13.3; SROCC SPM B.4'' }

'''Sea level rise is unavoidable for centuries to millennia due to continuing deep ocean warming and ice sheet melt, and sea levels will remain elevated for thousands of years (''' '''''high confidence).''''' Global mean sea level rise will continue in the 21st century ( ''virtually certain'' ), with projected regional relative sea level rise within 20% of the global mean along two-thirds of the global coastline ( ''medium confidence'' ). The magnitude, the rate, the timing of threshold exceedances, and the long-term commitment of sea level rise depend on emissions, with higher emissions leading to greater and faster rates of sea level rise. Due to relative sea level rise, extreme sea level events that occurred once per century in the recent past are projected to occur at least annually at more than half of all tide gauge locations by 2100 and risks for coastal ecosystems, people and infrastructure will continue to increase beyond 2100 ( ''high confidence'' ). At sustained warming levels between 2°C and 3°C, the Greenland and West Antarctic ice sheets will be lost almost completely and irreversibly over multiple millennia ( ''limited evidence'' ). The probability and rate of ice mass loss increase with higher global surface temperatures ( ''high confidence'' ). Over the next 2000 years, global mean sea level will rise by about 2 to 3 m if warming is limited to 1.5°C and 2 to 6 m if limited to 2°C ( ''low confidence'' ). Projections of multi-millennial global mean sea level rise are consistent with reconstructed levels during past warm climate periods: global mean sea level was ''very likely'' 5 to 25 m higher than today roughly 3 million years ago, when global temperatures were 2.5°C to 4°C higher than 1850–1900 ( ''medium confidence'' ). Further examples of unavoidable changes in the climate system due to multi-decadal or longer response timescales include continued glacier melt ( ''very high confidence'' ) and permafrost carbon loss ( ''high confidence'' ). { ''WGI SPM B.5.2, WGI SPM B.5.3, WGI SPM B.5.4, WGI SPM C.2.5, WGI Box TS.4, WGI Box TS.9, WGI 9.5.1; WGII TS C.5; SROCC SPM B.3, SROCC SPM B.6, SROCC SPM B.9'' } . ( ''Figure 3.4'' )

'''The probability of low- likelihood outcomes associated with potentially very large impacts increases with higher global warming levels (''' '''''high confidence).''''' Warming substantially above the assessed ''very likely'' range for a given scenario cannot be ruled out, and there is ''high confidence'' this would lead to regional changes greater than assessed in many aspects of the climate system. Low-likelihood, high-impact outcomes could occur at regional scales even for global warming within the ''very likely'' assessed range for a given GHG emissions scenario. Global mean sea level rise above the ''likely'' range – approaching 2 m by 2100 and in excess of 15 m by 2300 under a very high GHG emissions scenario (SSP5-8.5) ( ''lowconfidence'' ) – cannot be ruled out due to deep uncertainty in ice-sheet processes '''[[#footnote-034|123]]''' and would have severe impacts on populations in low elevation coastal zones. If global warming increases, some compound extreme events '''[[#footnote-033|124]]''' will become more frequent, with higher likelihood of unprecedented intensities, durations or spatial extent ( ''high confidence'' ). The Atlantic Meridional Overturning Circulation is ''very likely'' to weaken over the 21st century for all considered scenarios ( ''high confidence'' ), however an abrupt collapse is not expected before 2100 ( ''medium confidence'' ). If such a low probability event were to occur, it would ''very likely'' cause abrupt shifts in regional weather patterns and water cycle, such as a southward shift in the tropical rain belt, and large impacts on ecosystems and human activities. A sequence of large explosive volcanic eruptions within decades, as have occurred in the past, is a low-likelihood high-impact event that would lead to substantial cooling globally and regional climate perturbations over several decades. { ''WGI SPM B.5.3, WGI SPM C.3, WGI SPM C.3.1, WGI SPM C.3.2, WGI SPM C.3.3, WGI SPM C.3.4, WGI SPM C.3.5, WGI Figure SPM.8, WGI Box TS.3, WGI Figure TS.6, WGI Box 9.4; WGII SPM B.4.5, WGII SPM C.2.8; SROCC SPM B.2.7'' } . ( ''Figure 3.4, Cross-SectionBox.2'' )

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